In recent years, needs for colorization has been increasing in image forming devices using an electrophotographic method such as combined machines, facsimile machines and printers. In color printing, printing of an image for which reproduction of a color tone that has a high definition and is sharp like a photograph is also carried out, and thus a color toner that can correspond to the image is required. Furthermore, for such toner, there are needed various printing performances such as environmental stability in view of prevention of deterioration of an image quality due to environmental changes such as temperature and humidity, printing durability in view of decreasing of printing costs, and a low-temperature fixability in view of decreasing of power consumption.
In order to respond to the above-mentioned requirements, a spherical toner having a small particle diameter is suitable since the toner can attain both fine transfer property and dot reproducibility, and as a method for producing the toner, a polymerization method is suggested. In a conventional pulverizing process, specifically in the case when a toner having a small particle diameter is produced, the yield is low, and much energy is consumed in pulverization, whereas in a polymerization method, the yield is high, the consumed energy is low since the method does not require a pulverizing step, and a spherical toner can be easily produced.
As method for producing a toner by a polymerization method (hereinafter referred to as “polymerized toner”), a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method are exemplified. In the suspension polymerization method, firstly, a polymerizable monomer, a colorant, and where necessary, other additives are mixed to give a polymerizable monomer composition, and the composition is dispersed in an aqueous dispersion medium containing a dispersion stabilizer. Secondly, a high shear is applied to the aqueous dispersion medium in which the polymerizable monomer composition has been dispersed by using, for example, a high-speed agitator, such that droplets of the polymerizable monomer composition are formed. Thereafter, the aqueous dispersion medium in which the polymerizable monomer composition formed into droplets has been dispersed is polymerized in the presence of a polymerization initiator, and then subjected to filtration by a filtration material, washing and drying to give colored resin particles. Furthermore, external additives such as inorganic microparticles are mixed with these colored resin particles to give a polymerized toner.
In the case when colored resin particles are obtained by a polymerization method in such way, the method has great advantages that spherical colored resin particles can be formed with a smaller particle diameter than the particle diameter in a conventional pulverizing method in a stage of forming particles (it is a stage of forming droplets and conducting polymerization in the polymerization method, whereas it is a stage of conducting pulverization in the pulverizing method), and that a particle diameter distribution can be made sharper.
However, in recent years, in accordance with the further increasing of the required levels of high definition and high image quality, problems that should be solved are pointed out even in a polymerized toner.
In order to impart necessary friction chargeability to a negatively-chargeable toner, a charge control agent is generally added. As charge control agents, metal complexes of salicyclic acid or naphthoic acid with, for example, cobalt, chromium or iron, have been used. However, since charge control agents show high ionicity, there is a problem that a charge control agent is easily present in the vicinity of the surfaces of colored resin particles and thus, for example, fog easily occurs, in the production of a toner by a polymerization method.
In order to solve the above-mentioned problem, prevention of the presence of a charge control agent in the vicinity of the surfaces of colored resin particles more than required, by enhancing compatibilization with a binder resin by using a resin having negative chargeability (a negative-charging controlling resin) has also been suggested. Patent Literatures 1 to 3 each discloses that a resin having sulfonic acid group-containing monomer units within a specific range and having a weight average molecular weight within a specific range is utilized as a negative-charging controlling resin. Furthermore, Patent Literature 4 discloses that two negative-charging controlling resins having different sulfonic acid group-containing monomer units are used in combination.